1. Field of the Invention
The present invention relates to electrochemical storage devices, such as supercapacitors, batteries, etc., and more particularly to such devices that comprise an electrochemically active coaxial nanowire. The invention particularly concerns such devices in which the coaxial nanowire comprises an inner core of a transition metal oxide and an outer shell composed of an electroconductive organic polymer, such as poly(3,4-ethylenedioxythiophene) (PEDOT). The invention particularly relates to a facile method for achieving the self-assembly of such coaxial nanowires.
2. Description of Related Art
A. Electrochemical Supercapacitors
An electrochemical supercapacitor is an electrochemical energy storage device that provides high power while maintaining its energy density (or specific capacitance) at a high charge/discharge rate. To determine whether the above-described coaxial nanowires could be used to produce an electrochemical supercapacitor, the electrochemical properties of the coaxial nanowires were investigated.
Supercapacitors have received growing interest, with the increasing need for high-powered energy storage devices for electrical vehicles and mobile electronic devices (Winter et al. (2004) “What Are Batteries, Fuel Cells, and Supercapacitors?” Chem. Rev. 104:4245-4270; Burke, A. (2000) “Ultracapacitors: Why, How, And Where Is The Technology,” J. Power Sources 91:37-50; Vol'fkovich et al. (2002) “Electrochemical Capacitors,” Russ. J. Electrochem. 38:935-959). The supercapacitors work in conjunction with batteries to provide necessarily high peak power and enhance the life expectancy of the batteries. Based on their charge storage mechanism, supercapacitors are classified into two types: (i) an electrochemical double-layer capacitor (“EDLC”) (Pandolfo et al. (2006) “Carbon Properties And Their Role In Supercapacitors,” J. Power Sources 157:11-27) that stores the energy non-faradaically by charging an electrochemical double layer at the interface between the porous electrode and the electrolyte, and (ii) a redox supercapacitor (Conway et al. (1997) “The Role And Utilization Of Pseudocapacitance For Energy Storage By Supercapacitors,” J. Power Sources 66:1-14) that stores energy faradaically using the pseudocapacitance behaviour of a redox-active material. Studies have been focused on investigating the redox supercapacitors because they feature high energy densities (or specific capacitances).
Conductive polymers (Rudge et al. (1994) “Conducting Polymers As Active Materials In Electrochemical Capacitors,” J. Power Sources 47:89-107; Conway, B. E. (1999) In: ELECTROCHEMICAL SUPERCAPACITORS: SCIENTIFIC FUNDAMENTALS AND TECHNOLOGICAL APPLICATIONS (New York: Plenum) p. 299; Song et al. (2006) “Redox-Active Polypyrrole Toward Polymer-Based Batteries,” Adv. Mater. 18:1764-1768) and transition metal oxides (Zheng et al. (1995) “Hydrous Ruthenium Oxide as an Electrode Material for Electrochemical Capacitors,” J. Electrochem. Soc. 142:2699-2703; Hu et al. (2003) “Nanostructures and Capacitive Characteristics of Hydrous Manganese Oxide Prepared by Electrochemical Deposition,” J. Electrochem. Soc. 150:A1079-A1082) are promising materials for a redox supercapacitor because they can be readily converted between oxidized (doped) and reduced (dedoped) states by switching the applied potentials. This conversion process involves the diffusion of counter-ions into/out of conductive polymer or metal oxide films to keep their electroneutrality, which is a fundamental characteristic of a redox capacitor. Conductive polymers have been intensively investigated as electrode materials for supercapacitors because of their excellent electrochemical reversibilities, fast switching between redox states, high conductivity in a doped state, mechanical flexibility, low toxicity, and low cost (Malinauskas et al. (2005) “Conducting Polymer-Based Nanostructurized Materials Electrochemical Aspects,” Nanotechnology 16:R51-R62; Arbizzani et al. (2001) “New Trends In Electrochemical Supercapacitors,” J. Power Sources 100:164-170). In particular, poly(3,4-ethylenedioxythiophene) (PEDOT) is perceived as a good candidate for a supercapacitor (Carlberg et al. (1997) “Poly(3,4-ethylenedioxythiophene) as Electrode Material in Electrochemical Capacitors,” J. Electrochem. Soc. 144 L61-L64; Ryu et al. (2004) “Poly(ethylenedioxythiophene) (PEDOT) As Polymer Electrode In Redox Supercapacitor,” Electrochim. Acta 50:843-847; Lota et al. (2004) “Capacitance Properties Of Poly(3,4-ethylenedioxythiophene)/Carbon Nanotubes Composites,” J. Phys. Chem. Solids 65:295-301; Li et al. (2005) “Application Of Ultrasonic Irradiation In Preparing Conducting Polymer As Active Materials For Supercapacitor,” Mater. Lett. 59:800-803) because of its high stability among other conductive polymers (Heywang et al. (1992) “Poly(alkylenedioxythiophene)s-New, Very Stable Conducting Polymers,” Adv. Mater. 4:116-118). To date, most of the studies on PEDOT-based supercapacitors have been focused on enhancing their specific capacitances. For example, Lota et al. achieved a high specific capacitance of about 150 F g−1 by using PEDOT/carbon nanotube composites (Lota et al. (2004) “Capacitance Properties Of Poly(3,4-ethylenedioxythiophene)/Carbon Nanotubes Composites,” J. Phys. Chem. Solids 65:295-301). Li et al. enhanced the specific capacitance of PEDOT from 72 to 100 F g−1 using sponge-like PEDOT structures synthesized under ultrasonic irradiation (Li et al. (2005) “Application Of Ultrasonic Irradiation In Preparing Conducting Polymer As Active Materials For Supercapacitor,” Mater. Lett. 59:800-803). Jang et al reported that a high specific capacitance (155-170 F g−1) of PEDOT was achieved by selective fabrication of PEDOT nanocapsules and mesocellular foams (Jang et al. (2006) “Selective Fabrication of Poly(3,4-ethylenedioxythiophene) Nanocapsules and Mesocellular Foams Using Surfactant-Mediated Interfacial Polymerization,” Adv. Mater. 18:354-358). Besides achieving high specific capacitance (high energy density), the issue of how to obtain high power from PEDOT as an electrode material for a supercapacitor also needs immediate attention because more and more modern electronic devices require not only high energy but also high power.
In a redox supercapacitor, the high power can be achieved by enhancing the charge/discharge rate for the redox reaction. However, the conversion between redox states is governed by the mass transfer of counter-ions (Ingram et al. (2004) “‘Ladder-Doped’ Polypyrrole: A Possible Electrode Material For Inclusion In Electrochemical Supercapacitors?” J. Power Sources 129:107-112). The difficulty in keeping pace with a fast charging/discharge process at high power demand will lead to inefficient utilization of the electrode material, i.e. a loss of usable energy. A similar problem exists in lithium ion batteries: the slow diffusion of the lithium ion in the solid phase of the electrode materials limits its rate capability (Aricò et al. (2005) “Nanostructured Materials For Advanced Energy Conversion And Storage Devices,” Nat. Mater. 4:366-377; Cho et al. (2005) “Nanotube-Based Ultrafast Electrochromic Display,” Adv. Mater. 17:171-175; Cho et al. (2005) “Electrochemical Synthesis and Fast Electrochromics of Poly(3,4-ethylenedioxythiophene) Nanotubes in Flexible Substrate,” Chem. Mater. 17:4564-4566; Xiao et al. (2007) “Controlled Electrochemical Synthesis of Conductive Polymer Nanotube Structures,” J. Am. Chem. Soc. 129:4483-4489; Hu et al. (2006) “Design and Tailoring of the Nanotubular Arrayed Architecture of Hydrous RuO2 for Next Generation Supercapacitors,” Nano Lett. 6:2690-2695). Because of the intrinsic structural characteristics of arrays of one-dimensional hollow nanotubular structures (Aricò et al. (2005) “Nanostructured Materials For Advanced Energy Conversion And Storage Devices,” Nat. Mater. 4:366-377; Cho et al. (2005) “Nanotube-Based Ultrafast Electrochromic Display,” Adv. Mater. 17:171-175; Cho et al. (2005) “Electrochemical Synthesis and Fast Electrochromics of Poly(3,4-ethylenedioxythiophene) Nanotubes in Flexible Substrate,” Chem. Mater. 17:4564-4566; Xiao et al. (2007) “Controlled Electrochemical Synthesis of Conductive Polymer Nanotube Structures,” J. Am. Chem. Soc. 129:4483-4489; Hu et al. (2006) “Design and Tailoring of the Nanotubular Arrayed Architecture of Hydrous RuO2 for Next Generation Supercapacitors,” Nano Lett. 6:2690-2695), the use of such arrays has emerged as a possible solution for achieving a fast charge/discharge rate. The thin nature of the nanotube wall enables the rapid redox processes of electroactive materials such as conductive polymers and metal oxides by providing a short diffusion distance to the counter-ions. Furthermore, long nanotubes can provide high surface area and enough mass loading for electrode materials to store sufficient energy. Enhanced charge transport rates in template-synthesized one-dimensional nanomaterials have been reported (Van Dyke et al. (1990) “Electrochemical Investigations Of Electronically Conductive Polymers. 4. Controlling The Supermolecular Structure Allows Charge Transport Rates To Be Enhanced,” Langmuir 6:1118-1123; Cai et al. (1992) “Electrochemical Investigations Of Electronically Conductive Polymers VII. Charge Transport In Lightly Doped Polypyrrole,” Synth. Met. 46:165-179; Martin C. R. (1995) “Template Synthesis of Electronically Conductive Polymer Nanostructures,” Acc. Chem. Res. 28:61-68). For example, research has shown that higher lithium ion insertion rates could be achieved using nanofibres of vanadium pentoxide (Patrissi et al. (1999) “Sol-Gel-Based Template Synthesis and Li-Insertion Rate Performance of Nanostructured Vanadium Pentoxide,” J. Electrochem. Soc. 146:3176-3180; Sides et al. (2005) “Nanostructured Electrodes and the Low-Temperature Performance of Li-Ion Batteries,” Adv. Mater. 17:125-128), tin oxide (Li et al. (2000) “A High-Rate, High-Capacity, Nanostructured Tin Oxide Electrode,” Electrochem. Solid-StateLett. 3:316-318; Li et al. (2001) “Nanomaterial-Based Li-Ion Battery Electrodes,” J. Power Sources 97/98:240-243), and LiFePO4/carbon composite (Sides et al. (2005) “A High-Rate, Nanocomposite LiFePO4/Carbon Cathode,” Electrochem. Solid-State Lett. 8:A484-A487) and nanotubes of LiMn2O4 (Li et al. (2000) “Rate Capabilities of Nanostructured LiMn2O4 Electrodes in Aqueous Electrolyte,” J. Electrochem. Soc. 147:2044-2049) and TiS2 (Che et al. (1997) “Chemical-Vapor Deposition-Based Template Synthesis of Microtubular TiS2 Battery Electrodes,” J. Electrochem. Soc. 144:4296-4302). Fast switching between coloured and decoloured states of PEDOT can be found in our recent development of a nanotubebased devices (Cho et al. (2005) “Nanotube-Based Ultrafast Electrochromic Display,” Adv. Mater. 17:171-175; Cho et al. (2005) “Electrochemical Synthesis and Fast Electrochromics of Poly(3,4-ethylenedioxythiophene) Nanotubes in Flexible Substrate,” Chem. Mater. 17:4564-4566), that are also related to the fast charge/discharge rate. However, the application of coaxial PEDOT nanowires as supercapacitor electrode materials has not been studied before.
B. Nanowire Materials
A major challenge of the 21st century lies in the development of low-cost and environmentally friendly rechargeable energy storage systems (Arico, A. S. et al. (2005) “Nanostructured Materials For Advanced Energy Conversion And Storage Devices,” Nat. Mater. 4:366-377). Lithium ion batteries currently comprise preferred energy storage systems. In a typical lithium ion battery, the negative electrode (anode) comprises a lithium-storing metal (e.g., alloys of lithium and aluminum, silicon or tin). The anode is separated from the positive electrode (typically a lithium metal oxide) by a lithium ion-conducting electrolyte. When a lithium ion battery is discharging, lithium is extracted from the anode and inserted into the cathode. When the battery is charging, the reverse process occurs: lithium is extracted from the cathode and inserted into the anode. Unfortunately, the process of lithium insertion into the anode is associated with significant volume changes which strain and thus limit the useful life of the battery.
One proposed solution to this dilemma involves the use of nano-sized metallic clusters as the anode material (Huggins, R. A. (1999) “Lithium Alloy Anodes,” In: HANDBOOK OF BATTERY MATERIALS (Bernhard, J. O., Ed.), Part III, pp. 359-382; Wiley-VDCH, Weinheim); Winter, M. et al. (1999) “Electrochemical Lithiation Of Tin And Tin-Based Intermetallics And Composites,” Electrochim. Acta 45:31-50; Nazar, L. F. et al. (2004) “Anodes and Composite Anodes: An Overview,” In: LITHIUM BATTERIES SCIENCE AND TECHNOLOGY, (Nazri, G.-A. et al., Eds.), pp. 112-143; Kluwer Academic/Plenum, Boston). Unfortunately, such materials exhibit potentially significant side reactions, and are difficuly to produce with uniformity. Accordingly, they have not been fully satisfactory (Arico, A. S. et al. (2005) “Nanostructured Materials For Advanced Energy Conversion And Storage Devices,” Nat. Mater. 4:366-377).
Kim et al. (U.S. Pat. No. 7,084,002) describes a nano-structured electrode that comprises a metal oxide (MnO2) electrode that is substantially or completely free of an electroconductive organic polymer.
One-dimensional (1D) nanostructured materials have been intensively investigated as building components in electrochemical energy storage devices (Arico, A. S. et al. (2005) “Nanostructured Materials For Advanced Energy Conversion And Storage Devices,” Nat. Mater. 4:366-377; Patrissi, C. J. et al. (1999) “Sol-Gel-Based Template Synthesis and Li-Insertion Rate Performance of Nanostructured Vanadium Pentoxide,” J. Electrochem. Soc. 146:3176-3180; Hu, C. C. et al. (2006) “Design and Tailoring of the Nanotubular Arrayed Architecture of Hydrous RuO2 for Next Generation Supercapacitors,” Nano Lett. 6:2690-2695; Li, Q. G. et al. (2004) “Nanocrystalline α-MnO2 Nanowires by Electrochemical Step-Edge Decoration,” Chem. Mater. 16:3402-3405) and in solar energy conversion devices (Law, M. et al. (2005) “Nanowire Dye-Sensitized Solar Cells,” Nat. Mater. 4:455-459; Goodey, A. P. et al. (2007) “Silicon Nanowire Array Photelectrochemical Cells,” J. Am. Chem. Soc. 129:12344-12345) because they provide short diffusion path lengths to ions and excitons, leading to high charge/discharge rates.
More recently, coaxial nanowires have attracted greater attention in this field due to their added synergic properties (e.g., high conductivity) (Kim, D. W. et al. (2007) “Highly Conductive Coaxial SnO2—In2O3 Heterostructured Nanowires for Li Ion Battery Electrodes,” Nano Lett. 7:3041-3045) or functionalities (e.g., core/shell p-n junction) (Kovtyukhova, N. L. et al. (2005) “Nanowire p-n Heterojunction Diodes Made by Templated Assembly of Multilayer Carbon-Nanotube/Polymer/Semiconductor-Particle Shells around Metal Nanowires,” Adv. Mater. 17:187-192; Tian, B. Z. et al. (2007) “Coaxial Silicon Nanowires As Solar Cells And Nanoelectronic Power Sources,” Nature 449:885-890) arising from the combination of different materials (Kim, D. W. et al. (2007) “Highly Conductive Coaxial SnO2—In2O3 Heterostructured Nanowires for Li Ion Battery Electrodes,” Nano Lett. 7:3041-3045; Kovtyukhova, N. L. et al. (2005) “Nanowire p-n Heterojunction Diodes Made by Templated Assembly of Multilayer Carbon-Nanotube/Polymer/Semiconductor-Particle Shells around Metal Nanowires,” Adv. Mater. 17:187-192; Tian, B. Z. et al. (2007) “Coaxial Silicon Nanowires As Solar Cells And Nanoelectronic Power Sources,” Nature 449:885-890; Mieszawska, A. J. et al. (2007) “The Synthesis and Fabrication of One-Dimensional Nanoscale Heterojunctions,” Small 3:722-756; Wang, Y. et al. (2006) “Nanostructured Vanadium Oxide Electrodes for Enhanced Lithium-Ion Intercalation,” Adv. Funct. Mater. 16:1133-1144; Fan, H. J. et al. (2006) “Monocrystalline Spinel Nanotube Fabrication Based On The Kirkendall Effect,” Nat. Mater. 5:627-631; Liu, Z. Q. et al. (2005) “Single Crystalline Magnetite Nanotubes,” J. Am. Chem. Soc. 127:6-7).
Various materials such as semiconductor/semiconductor, metal/metal oxide, and metal oxide/metal oxide, have been employed as core/shell in coaxial nanowires (Kim, D. W. et al. (2007) “Highly Conductive Coaxial SnO2—In2O3 Heterostructured Nanowires for Li Ion Battery Electrodes,” Nano Lett. 7:3041-3045; Kovtyukhova, N. L. et al. (2005) “Nanowire p-n Heterojunction Diodes Made by Templated Assembly of Multilayer Carbon-Nanotube/Polymer/Semiconductor-Particle Shells around Metal Nanowires,” Adv. Mater. 17:187-192; Tian, B. Z. et al. (2007) “Coaxial Silicon Nanowires As Solar Cells And Nanoelectronic Power Sources,” Nature 449:885-890; Mieszawska, A. J. et al. (2007) “The Synthesis and Fabrication of One-Dimensional Nanoscale Heterojunctions,” Small 3:722-756; Wang, Y. et al. (2006) “Nanostructured Vanadium Oxide Electrodes for Enhanced Lithium-Ion Intercalation,” Adv. Funct. Mater. 16:1133-1144; Fan, H. J. et al. (2006) “Monocrystalline Spinel Nanotube Fabrication Based On The Kirkendall Effect,” Nat. Mater. 5:627-631; Liu, Z. Q. et al. (2005) “Single Crystalline Magnetite Nanotubes,” J. Am. Chem. Soc. 127:6-7).
Despite all such prior advances, a need remains for energy storages systems capable of use in modern electronic devices and particularly for such devices that can be readily formed and which do not require complex and/or multi-step synthesis. The present invention is directed to this and other needs.